Waveguide in multilayer structures

ABSTRACT

The idea of the invention is to fabricate a multilayer coaxial transmission line into a printed circuit. The outermost conductor is fabricated by conductive strips in different layers, using conductive via posts in isolation layers connecting the strips. The innermost conductor can be a single conductive strip or multi strips in different layers connected together through conductive via posts.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/297,763,filed Dec. 9, 2002, which is a 371 of PCT/FI/00482, filed May 17, 2001,which claims priority from Finnish application 20001383, filed May 17,2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to waveguides, transmission lines, and resonatorsin multilayer printed circuit board structures. In particular theinvention concerns a coaxial line structure in printed circuits.

2. Background of the Invention

Prior art transmission lines used in printed circuits have mainly beenmicrostrips, coplanar waveguides, and striplines. If a coaxial line havebeen needed in a circuit board, the coaxial has been attached separatelyto the surrounding circuitry on the surface of the printed circuit. Thiscreates a potential reliability risk due to the increased number ofjoints, additional electrical loss, and temperature growth related todiscontinuity effects. Manufacturing is expensive because of thediscrete nature of a coaxial line. Coaxial structures are alsorelatively large because the dielectric material in the coaxial has alow dielectric constant (ε_(r)=2-4). Conventional coaxial structuresalso need space for supporting and protective layers around the cable.

As mentioned, microstrips, coplanar waveguides, and striplines are theusual structures in multilayer printed circuits as mentioned. These aresensitive to EMC disturbances, such as electromagnetic interference. Dueto the effect of radiation, especially at high frequencies, conventionalmultilayer transmission lines have high transmission losses as well.Using typical multilayer transmission line structures means that onlymoderate Q values can be gained, making it difficult to apply multilayerceramics technologies in high Q value applications, such as resonators.

The objective of the invention is to decrease the above-mentioneddrawbacks of known solutions.

SUMMARY OF THE INVENTION

The idea of the invention is to fabricate a multilayer coaxialtransmission line into a printed circuit. The outermost conductor isfabricated by conductive conduit strips in different layers, usingconductive via posts in isolation layers connecting the strips. Theinnermost conductor can be a single conductive strip or multiple stripsin different layers connected together through conductive via posts. Theobjective of the invention is achieved in a manner described in theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is described in more detail by means ofthe attached figures, of which

FIG. 1 illustrates an example of a coaxial structure according to theinvention,

FIG. 2 illustrates an example of a conductive via post structure,

FIG. 3 illustrates an example of a cross-section of a coaxial structureaccording to the invention,

FIG. 4 illustrates another example of a cross-section of a coaxialstructure according to the invention,

FIG. 5 shows an example of a diagram of a reflection coefficient in acoaxial structure according to FIG. 4,

FIG. 6 shows an example of a diagram of a reflection coefficient in acoaxial structure according to FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts an example of the coaxial structure (10) according to theinvention. The innermost line strip (1) of the multilayer structureforms a transmission line for a signal, and the outermost line strips(2) in different layers of a circuit board, which are connected by viaposts (3) to each other, form a ground conductor. The outermost linestrips are arranged to form as round a circle as possible in order toimitate the actual coaxial waveguide. The distance (4) between the viaposts should be less than a quarter in wavelength in order to guaranteegood isolation between the coaxial structure and the rest of the circuitboard. The via post separation (4) must not be more than half awavelength, since otherwise multiple transmission lines start to behavelike coupled resonators, resulting in severe oscillations within thestructure. The minimum length between via posts is determined by themechanical strength of a circuit board structure.

In multilayer ceramics technology, it is easy to form via posts inisolation, which can be filled with conductive paste to form signalpaths before the firing process. By using a screen printing process, itis easy to create conductive transmission line strips as well. Via postsneed not be round taps, but they can be other shapes too. FIG. 2illustrates via posts which are rectangles. The post can be placed inmore than one row in order to improve isolation.

In FIG. 3 another example of the cross-section of a coaxial structure isdepicted according to the invention. Now the signal conductor iscomposed of two line strips in different layers, which are connectedwith via posts through isolation (5). This is to suppress conductorlosses. There are also two outermost conductors in this case, i.e. FIG.3 represents a triaxial structure. The triaxial structure is especiallyneeded, for example, in sensitive measurement devices or if goodimmunity to EMI disturbances is otherwise required. It is also possibleto fabricate more conductors in a coaxial structure if needed, i.e. toform multiaxial structures.

The short-circuit ending of a coaxial structure is formed by usingconductive strips (6) (see FIG. 1), posts (7), or both, to connect theinnermost conductor to the outermost.

FIG. 4 shows the cross-section of a coaxial structure where theoutermost conductor is in the form of rectangle. The rectangle form iseasier to manufacture and more inexpensive than the structure where theoutermost conductor is in the form of round. However, losses are greaterin the form of rectangle. FIG. 5 shows an example of a reflectioncoefficient (S) when the test signal is input to the rectangular coaxialline (FIG. 4) whose the other end has been supplied with ashort-circuit. Correspondingly, FIG. 6 shows an example of a reflectioncoefficient when the test signal is input to the round coaxial line(FIG. 1) whose the other end has been supplied with a short-circuit. Thereflection coefficient is greater in the round coaxial line than in therectangular coaxial line, i.e. there are more losses in the rectangularcoaxial line. Thus it is usually desired to arrange the outermost linestrips for forming as round a circle as possible.

Because the multilayer coaxial structure is integrated into a printedcircuit, this is an inexpensive solution. There is more space on thesurface of the printed circuit for other components. The size of thecoaxial structure can be reduced if high dielectric constant material isused. Typically dielectric materials used in coaxial cables have lowdielectric constant values (2 . . . 4), while in multilayer ceramicsdielectric constant values range from 5 up to several hundred.

EMC problems can be avoided since the outermost line of a coaxial linecan act as an EMC shield to the surrounding circuitry. This also meanslow radiation losses. The coaxial structure has inherently a high Qvalue due to its geometry. High Q value of a multilayer coaxialstructure enables the usage of high Q value resonators and filters inmultilayer circuit boards.

The coaxial line has a low dispersion since it is of the TEM (TransverseElectric-Magnetic) transmission line type, providing that the use ofthis kind of transmission line is possible at high frequencies.

The invention could be applied, for example, to either multilayerceramic technologies, such as LTCC (Low Temperature Cofired Ceramics)and HTCC (High Temperature Cofired Ceramics), or to advanced laminatedprinted circuit board technology.

Although the invention has been described in the light of theabove-mentioned examples, it is evident that the invention is notrestricted to them, but that it can be used in other structures withinthe scope of the inventive idea.

1. A circuit board transmission line, comprising: a first conductorincluding a plurality of first line strips disposed along a lengthwisedirection on different layers of a circuit board, said plurality offirst line strips being connected by conductive via posts to provide aconductive pipe having a round cross-section; and a second conductorincluding at least one second line strip disposed along the lengthwisedirection into a center of the first conductor, wherein a set of viaposts are provided in a line and a distance between adjacent via postsin the line is less than a quarter of a wavelength of a signal for whichthe circuit board transmission line is designed.
 2. The circuit boardtransmission line according to claim 1, the second conductor comprisinga plurality of second line strips in different layers of the circuitboard, said plurality of second line strips being connected together byfurther conductive via posts.
 3. The circuit board transmission lineaccording to claim 1, further comprising additional conductors of a sametype as the first conductor.
 4. The circuit board transmission lineaccording to claim 1, further comprising conductive elements connectingthe first conductor to the second conductor.
 5. The circuit boardtransmission line according to claim 1, wherein the conductive via postsbetween the plurality of first line strips in the first conductor aredisposed in a row parallel to the plurality of first line strips.
 6. Thecircuit board transmission line according to claim 1, wherein theconductive via posts between the plurality of first line strips in thefirst conductor are disposed in several rows parallel to the pluralityof first line strips.
 7. The circuit board transmission line accordingto claim 1, wherein the conductive via posts comprise round taps.
 8. Thecircuit board transmission line according to claim 1, wherein theconductive via posts comprise rectangular taps.
 9. A resonatorcomprising: a first conductor including a plurality of line stripsdisposed along a lengthwise direction on different layers of the circuitboard, said plurality of line strips being connected by conductive viaposts to provide a conductive pipe having a round cross-section; and asecond conductor disposed along the lengthwise direction into a centerof the first conductors wherein a set of via posts are provided in aline and a distance between adjacent via posts in the line is less thana quarter of a wavelength of a signal for which the circuit boardtransmission line is designed.
 10. The resonator according to claim 9,wherein the second conductor comprises a line strip.
 11. The resonatoraccording to claim 9, further comprising conductive elements between thefirst conductor and the second conductor.
 12. The resonator according toclaim 9, wherein the second conductor has a cross-sectional structuresimilar to that of the first conductor.
 13. The resonator according toclaim 9, wherein the conductive via posts between adjacent pairs of theplurality of line strips of the first conductor are disposed in a rowparallel to the plurality of line strips.
 14. The resonator according toclaim 9, wherein the conductive via posts between adjacent pairs of theplurality of line strips of the first conductor are disposed in severalrows parallel to the plurality of line strips.
 15. The resonatoraccording to claim 9, wherein the conductive via posts comprise roundtaps.
 16. The resonator according to claim 9, wherein the conductive viaposts comprise rectangular taps.
 17. A method of manufacturing a coaxialstructure, the method comprising the steps of: providing a plurality offirst line strips arranged in successive layers on a circuit board toform a hollow member by arranging said plurality of first line strips ina shape of a circle; connecting the plurality of first line strips toeach other by via posts, providing a set of via posts in a line, adistance between adjacent via posts in the line being less than aquarter of a wavelength of a signal for which the circuit boardtransmission line is designed; inserting at least one second line stripin a center of the plurality of first line strips; and mounting the atleast one second line strip to the plurality of first line strips by atleast one connecting member.
 18. The method according to claim 17,further comprising the step of filling the via posts with a conductivepaste to form signal paths.
 19. The method according to claim 17,wherein the step of mounting the at least one second line strip to theplurality of first line strips comprises mounting the at least onesecond line strip by at least one conductive strip.
 20. The methodaccording to claim 17, wherein the step of mounting the at least onesecond line strip to the plurality of first line strips comprisesmounting the at least one second line strip by at least one post. 21.The method according to claim 17, wherein the step of mounting the atleast one second line strip to the plurality of first line stripscomprises mounting the at least one second line strip by a conductivestrip and a post.
 22. A coaxial structure comprising: a plurality offirst conducting means for conducting a current arranged in successivelayers on a circuit board to provide a hollow member comprising acircular cross section; connecting means for connecting the plurality offirst conducting means to each other; second conducting means forconducting a current disposed in a hollow center of the plurality of thefirst conducting means; and mounting means for mounting the secondconducting means to the first conducting means, wherein a set of viaposts are provided in a line and a distance between adjacent via postsin the line is less than a quarter of a wavelength of a signal for whichthe circuit board transmission line is designed.
 23. The coaxialstructure according to claim 22, wherein the via posts are filled withconductive paste.
 24. The coaxial structure according to claim 22,wherein the mounting means comprises conductive strips.
 25. The coaxialstructure according to claim 22, wherein the mounting means comprises atleast one conductive strip and at least one conductive post.